The 7-digit numbers 7123A32, 2B25689 and 450ABC6 are divisible by 3. Find the sum of all possible values of C.

9514 1404 393

  15

Step-by-step explanation:

For numbers to be divisible by 3, the sum of their digits must be divisible by 3.

This means we require ...

  (7 + 1 + 2 + 3 + A + 3 + 2) mod 3 = 0   ⇒   A mod 3 = 0

  (2 + B + 2 + 5 + 6 + 8 + 9) mod 3 = 0   ⇒   (2+B) mod 3 = 0   ⇒   B mod 3 = -2

  (4 + 5 + 0 + A + B + C + 6) mod 3 = 0   ⇒   (C -2) mod 3 = 0

Possible values of C are 2, 5, 8. Their sum is 15.

For example, let A=0, B=1, C=2. Then we have ...

  7123032 = 2374344×3

  2125689 = 708563×3

  4500126 = 1500042×3

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